The purpose of this feature is to facilite exposing the reset-state API in
horizon in a meaningful way by restricting the set of permissible states that
the administrator can specify for a volume. There is no API for this, and it
is undesirable to hardcode this information into horizon.

For consistency with the rest of cinder, get_valid_states may be renamed and/or
split into multiple functions, one per resource type; this decision will be
left as an implementation detail and will be finalized as part of the normal
code review process.